PROJECT SUMMARY/ABSTRACT Inflammatory bowel disease (IBD) is characterized by severe gastrointestinal inflammation and changes in the intestinal microbiota. Many IBD patients also experience extra-intestinal manifestations. Skeletal abnormalities are a frequent extra-intestinal manifestation of IBD, and patients exhibit up to a 40% increased risk of incurring fractures compared to the general population. Although many factors, including malabsorptive malnutrition and glucocorticoid use, contribute to IBD-associated bone loss, nutritionally replete and glucocorticoid naive patients remain susceptible to bone loss. While changes in both the intestinal microbiome and inflammatory cytokines occur in IBD, the precise mechanisms of IBD-associated bone loss are unclear. We have therefore established complementary mouse models of colitis-driven bone loss, and are uniquely positioned to use skeletal specific genetic knockout mice to define inflammatory and microbiotic drivers of IBD-associated bone loss. My preliminary data implicate the IL-12/23 signaling axis in the pathogenesis of bone loss during colitis. Intriguingly, these cytokines share a common subunit, IL-12/23p40 (p40), yet exert opposing effects on bone remodeling. IL-12 inhibits osteoclast differentiation, while IL-23 leads to bone resorption through enhanced osteoclastogenesis. Dual blockade of IL-12/23 is used clinically via monoclonal p40 antibodies; however, the effect of p40 blockade on bone remodeling during gastrointestinal inflammation is unclear. In addition to the impact of cytokines on skeletal homeostasis, emerging evidence has identified the intestinal microbiome as a regulator of bone remodeling. Many studies have characterized changes in the IBD intestinal microbiome and demonstrated that these changes are transmitted systemically by crossing the gut epithelium. Importantly, skeletal cells sense and respond to microbial components through pattern recognition receptors, many of which are dependent on the adaptor protein, MyD88. The overarching hypothesis of this proposal is that 1) IL-12/23 signaling and 2) circulating microbial components impact skeletal homeostasis during gastrointestinal inflammation. I will test this hypothesis with two complementary Specific Aims. In Specific Aim 1, I will define the role of IL-12/23 signaling on direct and indirect osteoclastogenesis during colitis. In Specific Aim 2, I will determine the impact of skeletal MyD88-dependent PRRs on colitis-associated bone loss. Successful completion of the proposed experiments will elucidate fundamental mechanisms by which systemic cytokines and circulating microbiota components impact bone biology during colitis and provide a platform to test the role of alternative colitis-associated cytokines in pathologic bone loss. Furthermore, these studies will clarify how therapeutic IL- 12 and IL-23 dual blockade in patients with IBD impacts skeletal homeostasis. Collectively, this proposal will investigate how the IL-12/23 signaling axis and the microbiome impact skeletal homeostasis during gastrointestinal inflammation while providing the ideal training and mentorship to develop my future career as an independent, NIH-funded pediatric physician-scientist.